Calpains are intracellular, proteolytic enzymes from the cysteine protease group and are found in many cells. The enzyme calpain is activated by elevated calcium concentration, with a distinction being made between calpain I or μ-calpain, which is activated by μ-molar concentrations of calcium ions, and calpain II or m-calpain, which is activated by m-molar concentrations of calcium ions. Currently, further calpain isoenzymes are also postulated (M. E. Saez et al.; Drug Discovery Today 2006, 11 (19/20), pp. 917-923; K. Suzuki et al., Biol. Chem. Hoppe-Seyler, 1995, 376 (9), pp. 523-9).
Calpains play an important role in various physiological processes. These processes include the cleavage of different regulatory proteins such as protein kinase C, cytoskeletal proteins such as MAP 2 and spectrin, and muscle proteins, protein degradation in rheumatoid arthritis, proteins in the activation of platelets, neuropeptide metabolism, proteins in mitosis, and others which are listed in: M. J. Barrett et al., Life Sci. 1991, 48, pp. 1659-69; K. Wang et al., Trends in Pharmacol. Sci. 1994, 15, pp. 412-419.
Elevated calpain levels have been measured in various pathophysiological processes, for example: ischemias of the heart (e.g. myocardial infarction), the kidney, the lung, the liver or the central nervous system (e.g. stroke), inflammations, muscular dystrophies, cataracts of the eyes, diabetes, HIV disorders, injuries to the central nervous system (e.g. brain trauma), Alzheimer's, Huntington's, Parkinson's diseases, multiple sclerosis etc. (see K. K. Wang, above) and infectious diseases such as malaria (I M Medana et al., Neuropath and Appl. Neurobiol. 2007, 33, pp. 179-192). It is assumed that there is a connection between these diseases and generally or persistently elevated intracellular calcium levels. This results in calcium-dependent processes becoming hyperactivated and no longer being subject to normal physiological control. A corresponding hyperactivation of calpains can also trigger pathophysiological processes.
For this reason, it was postulated that inhibitors of calpain could be of use for treating these diseases. This postulate was confirmed by a variety of investigations. Thus, Seung-Chyul Hong et al., Stroke 1994, 25 (3), pp. 663-669, and R. T. Bartus et al., Neurological Res. 1995, 17, pp. 249-258, have demonstrated that calpain inhibitors have a neuroprotective effect in acute neurodegenerative impairments or ischemias such as occur after cerebral stroke. K. E. Saatman et al., Proc. Natl. Acad. Sci. USA, 1996, 93, pp. 3428-3433 describe that following experimental brain trauma, calpain inhibitors also improved recovery from the memory performance deficits and neuromotor impairments. C. L. Edelstein et al., Proc. Natl. Acad. Sci. USA, 1995, 92, pp. 7662-6, found that calpain inhibitors have a protective effect on hypoxia-damaged kidneys. Yoshida, Ken Ischi et al., Jap. Circ. J. 1995, 59 (1), pp. 40-48, pointed out that calpain inhibitors had favorable effects following cardiac damage which was produced by ischemia or reperfusion. The calpain inhibitor BDA-410 delayed the progression of malaria infection in a mouse model of malaria pathogenesis as shown by X. Li et al., Mol. Biochem. Parasitol. 2007, 155 (1), pp 26-32.
More recent studies have shown in calpastatin transgenic animals that the expression of the natural inhibitor of calpain significantly attenuates the pathophysiological effects of activated calpain in experimental glomerulonephritis shown by J. Peltier et al., J. Am. Soc. Nephrol. 2006, 17, pp. 3415-3423, in cardiovascular remodeling in angiotensin II-induced hypertension, in impaired synaptic transmission in slow-channel congenital myasthenic syndrome shown by J. S. Groshong et al., J. Clin. Invest. 2007, 117(10), pp 2903-2912, in excitotoxic DNA fragmentation via mitochondrial pathways shown by J. Takano et al., J. Biol. Chem. 2005, 280 (16), pp. 16175-16184, and in necrotic processes in dystrophic muscles shown by M. J. Spencer et al., Hum. Mol. Gen., 2002, 11(21), pp. 2645-2655.
It has been shown in recent years that both the function and the metabolism of a number of important proteins involved in the development of Alzheimer's disease are modulated by calpain. Various external influences such as, for example, excitotoxins, oxidative stress or else the action of amyloid protein lead to hyperactivation of calpain in the nerve cell, causing, as cascade, a dysregulation of the CNS-specific kinase cdk5 and subsequently a hyperphosphorylation of the so-called tau protein. Whereas the actual task of the tau protein consists of stabilizing the microtubules and thus the cytoskeleton, phosphorylated tau is no longer able to fulfil this function; the cytoskeleton collapses, axonal transport of matter is impaired and thus eventually the nerve cell degenerates (G. Patrick et al., Nature 1999, 402, pp. 615-622; E. A. Monaco et al.; Curr. Alzheimer Res. 2004, 1 (1), pp. 33-38). Accumulation of phosphorylated tau additionally leads to the formation of so-called neurofibrillary tangles (NFTs) which, together with the well-known amyloid plaques, represent a pathological hallmark of Alzheimer's disease. Similar changes in the tau protein, generally referred to important feature of as tauopathies are also observed in other (neuro)degenerative disorders such as, for example, following stroke, inflammations of the brain, Parkinsonism, in normal-pressure hydrocephalus and Creutzfeldt-Jakob disease.
The involvement of calpain in neurodegenerative processes has been demonstrated in transgenic mice with the aid of calpastatin, a specific and natural inhibitor of calpains (Higuchi et al.; J. Biol. Chem. 2005, 280 (15), pp. 15229-15237). It was possible with the aid of a calpain inhibitor to reduce markedly the clinical signs of acute autoimmune encephalomyelitis in a mouse model of multiple sclerosis (F. Mokhtarian et al.; J. Neuroimmunology 2006, Vol. 180, pp. 135-146). It has further been shown that calpain inhibitors on the one hand block the Ab-induced degeneration of neurons (Park et al.; J. Neurosci. 2005, 25, pp. 5365-5375), and in addition reduce the release of the β-amyloid precursor protein (β APP) (J. Higaki et al., Neuron, 1995, 14, pp. 651-659). With this background, calpain inhibitors having sufficient CNS availability represent a novel therapeutic principle for the treatment of neurodegenerative disorders in general and in particular also of Alzheimer's disease.
The release of interleukin-1α is likewise inhibited by calpain inhibitors (N. Watanabe et al., Cytokine 1994, 6(6), pp. 597-601). It has additionally been found that calpain inhibitors show cytotoxic effects on tumor cells (E. Shiba et al. 20th Meeting Int. Ass. Breast Cancer Res., Sendai Jp, 1994, 25.-28. Sep., Int. J. Oncol. S(Suppl.), 1994, 381).
The involvement of calpain in HIV disorders has only recently been shown. Thus, it has been demonstrated that the HIV-induced neurotoxicity is mediated by calpain (O'Donnell et al.; J. Neurosci. 2006, 26 (3), pp. 981-990). Calpain involvement in the replication of the HIV virus has also been shown (Teranishi et al.; Biochem. Biophys. Res. Comm. 2003, 303 (3), pp. 940-946).
Recent investigations indicate that calpain plays a part in so-called nociception, the perception of pain. Calpain inhibitors showed a distinctly beneficial effect in various preclinically relevant models of pain, e.g. in the thermally induced hyperalgesia in rats (Kunz et al.; Pain 2004, 110, pp. 409-418), in Taxol-induced neuropathy (Wang et al.; Brain 2004, 127, pp. 671-679) and in acute and chronic inflammatory processes (Cuzzocrea et al.; American Journal of Pathology 2000, 157 (6), pp. 2065-2079).
The involvement of calpain in the development of kidney diseases, such as chronic kidney diseases, e.g. diabetic nephropathy, has also been shown recently. Thus, it has been demonstrated by Y. Shi et al. in animal models that the natural calpain inhibitor calpastatin is down regulated during renal ischemia reperfusion (Am. J. Physiol. Renal Physiol. 2000, 279, pp. 509-517). Furthermore, A. Dnyanmote et al., Toxicology and Applied Pharmacology 2006, 215, pp. 146-157, have shown that inhibition of calpain via overexpression of calpastatin reduces the progression of DCVC-induced renal injury in a model of acute renal failure. In addition, Peltier et al. have demonstrated that calpain activation and secretion promotes glomerular injury in experimental glomerulonephritis (J. Am. Soc. Nephrol. 2006, 17, pp. 3415-3423). It has also been shown that calpain inhibitors reduce renal dysfunction and injury caused by renal ischemia-reperfusion and thus may be useful in enhancing the tolerance of the kidney against renal injury associated with aortovascular surgery or renal transplantation (P. Chatterjee et al., Biochem. Pharmacol. 2005, 7, pp. 1121-1131).
Further possible applications of calpain inhibitors are detailed in: M. E. Saez et al.; Drug Discovery Today 2006, 11 (19/20), pp. 917-923; N. O. Carragher, Curr. Pharm. Design 2006, 12, pp. 615-638; K. K. Wang et al.; Drugs of the Future 1998, 23 (7), pp. 741-749; and Trends in Pharmacol. Sci. 1994, 15, pp. 412-419.
With the calpain inhibitors described to date a general distinction is made between irreversible and reversible inhibitors, and peptide and non-peptide inhibitors.
Irreversible inhibitors are usually alkylating substances. They have the disadvantage that they firstly react unselectively and/or are unstable in the body. Thus, corresponding inhibitors often show unwanted side effects such as toxicity, and application thereof is therefore markedly restricted. The irreversible inhibitors include for example epoxides such as E64, α-halo ketones, and disulfides.
A large number of known reversible calpain inhibitors are peptide aldehydes which are derived in particular from di- or tripeptides such as, for example, Z-Val-Phe-H (MDL 28170). Derivatives and prodrugs structurally derived from aldehydes are also described, especially corresponding acetals and hemiacetals (e.g. hydroxytetrahydro-furans, hydroxyoxazolindines, hydroxymorpholines and the like), but also imines or hydrazones. However, under physiological conditions, peptide aldehydes and related compounds usually have the disadvantage that, owing to their reactivity, they are frequently unstable, are rapidly metabolized and are prone to unspecific reactions which may likewise cause toxic effects (J. A. Fehrentz and B. Castro, Synthesis 1983, pp. 676-78).
In recent years, a number of non-peptide carboxamides having a β-keto function in the amine moiety and inhibiting calpain have been described. Thus, WO-98/16512 describes 3-amino-2-oxo carboxylic acid derivatives whose amino group is amidated with a 4-piperidinecarboxylic acid compound. WO-99/17775 describes similar compounds which are amidated with a quinolinecarboxylic acid. WO-98/25883, WO-98/25899 and WO-99/54294 describe 3-amino-2-oxo carboxylic acid derivatives whose amino group is amidated with a substituted benzoic acid. WO-99/61423 describes 3-amino-2-oxo carboxylic acid derivatives whose amino group is amidated with an aromatic carboxylic acid carrying a tetrahydroquinoline/isoquinoline and 2,3-dihydroindole/isoindole residue. Similar compounds in which the aromatic carboxylic acid residue carries a heterocyloalkyl radical or (hetero)aryl radical which is optionally connected via a linker are described in WO-99/54320, WO-99/54310, WO-99/54304 and WO-99/54305. Likewise, WO 08/080969 describes nicotinamides of 3-amino-2-oxo carboxylic acid derivatives that in position 2 of the pyridine ring are linked to a substituted pyrazole via a nitrogen atom. WO-99/54293 describes benzamides of 4-amino-3-oxo carboxylic acid derivatives. WO-03/080182 describes the use of the aforementioned amides for the treatment of pulmonary diseases. The nonpeptide calpain inhibitors mentioned therein also have a number of disadvantages, in particular a low or absent selectivity in respect of related cysteine proteases, such as various cathepsins, likewise possibly leading to unwanted side effects.